The present invention relates to a battery protecting system in a car stereo for preventing battery voltage from decreasing below a predetermined value.
A car stereo mounted in a motor vehicle is supplied with electric power from a battery mounted in the motor vehicle when an ignition key is set to an accessory (ACC) position (FIG. 6) after stopping of the engine or an ON position during the operation of the engine. The battery mounted in the vehicle is charged during the operation of the engine.
If the ignition key is set to the ACC position and the car stereo is used for a long time, the voltage of the battery reduces, causing difficulty in starting of the engine, and shortening the life of the battery.
In a conventional motor vehicle, if the ignition key is set to an OFF position (including a LOCK position for pulling out the key), the electric power from the battery to the car stereo is cut off-key, the car stereo can not be operated and hence a passenger can not operate the car stereo.